Add metadata SHOW supports: `SHOW databases`/`SHOW tables`/`SHOW schemas`

[xiangfu0]

• Extend SQL grammar and execution to support metadata SHOW statements (databases/schemas/tables), wiring broker/controller dispatch and executor handling.
• Add unit coverage for parsing/execution along with a stubbed metadata executor test.
• Add integration test ensuring SHOW TABLES matches PinotAdminClient results and include pinot-java-client as test dependency.

[codecov-commenter]

Codecov Report

:x: Patch coverage is 47.76786% with 117 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 63.30%. Comparing base (624b90b) to head (ad5e4fe). :warning: Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
...inot/core/query/executor/sql/SqlQueryExecutor.java	49.27%	47 Missing and 23 partials :warning:
...pinot/broker/api/resources/PinotClientRequest.java	10.00%	8 Missing and 1 partial :warning:
...org/apache/pinot/broker/grpc/BrokerGrpcServer.java	38.46%	7 Missing and 1 partial :warning:
...pache/pinot/sql/parsers/parser/SqlShowSchemas.java	41.66%	7 Missing :warning:
...apache/pinot/sql/parsers/parser/SqlShowTables.java	41.66%	7 Missing :warning:
...che/pinot/sql/parsers/parser/SqlShowDatabases.java	37.50%	5 Missing :warning:
...t/controller/api/resources/PinotQueryResource.java	0.00%	5 Missing :warning:
...apache/pinot/client/admin/PinotAdminTransport.java	75.00%	1 Missing and 1 partial :warning:
...org/apache/pinot/sql/parsers/CalciteSqlParser.java	60.00%	1 Missing and 1 partial :warning:
...e/pinot/broker/broker/helix/BaseBrokerStarter.java	0.00%	1 Missing :warning:
... and 1 more

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17357      +/-   ##
============================================
+ Coverage     63.24%   63.30%   +0.05%     
+ Complexity     1474     1469       -5     
============================================
  Files          3147     3151       +4     
  Lines        187490   187789     +299     
  Branches      28703    28746      +43     
============================================
+ Hits         118575   118873     +298     
+ Misses        59728    59681      -47     
- Partials       9187     9235      +48     

Flag	Coverage Δ
custom-integration1	?
integration	0.00% <ø> (-100.00%)	:arrow_down:
integration1	?
integration2	0.00% <ø> (ø)
java-11	63.26% <47.76%> (+0.09%)	:arrow_up:
java-21	63.24% <47.76%> (+0.03%)	:arrow_up:
temurin	63.30% <47.76%> (+0.05%)	:arrow_up:
unittests	63.30% <47.76%> (+0.06%)	:arrow_up:
unittests1	54.85% <51.79%> (-0.80%)	:arrow_down:
unittests2	33.92% <12.94%> (+0.05%)	:arrow_up:

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